The present invention relates generally to radar systems and more particularly to radar specifically adapted for penetrating soil and similar obstructions.
In the prior art it has been the practice in ground penetrating radar systems to employ baseband video pulse radar technology. Unfortunately, although it would be desirable that the transmit and receive signal pulses of the radar be of similar shape and size, distortion occurs in the return in the form of a ringing effect, due to antenna characteristics, soil attenuation properties and the location of targets in the antenna's near field. This ringing phenomenon can be very complex and extremely difficult to decorrelate when trying to extract relevant information. In addition, in order to achieve adequate resolution, the system must use very fast pulses. This places a finite limit on the average power which, in turn, reduces the signal to noise ratio of the system. Another problem is the lack of phase information in such prior art systems which, if available, could yield an improvement in the radar return signal through he use of digital signal processing.
Among the more difficult problems which have been identified in such prior art systems are the unutilized low frequency components of the radar pulse signal, the lack of coherence, and the inherently low average power of such systems. In addition, the difficulty of building high dynamic range sampling hardware, the lack of a broadband, high isolation, fast transmit/receive switch, and the apparently impossible task of building an antenna to radiate the entire signal bandwidth, have indicated the need for an entirely different approach to providing a radar system for detecting anomalies in geophysical media.
The deficiencies of commercial radar systems have also been noted by others in the field. For example, U.S. Pat. No. 4,218,678, issued to Fowler et al., discloses an earth penetrating radar system having a transmitter section and a distinct receiver section. In the transmitter portion, a digital signal from a microprocessor and a base periodic signal from a master oscillator are both fed to a frequency synthesizer where the signals are multiplied to produce a base reference signal which passes through an attenuator control and an amplifier to a transmitter antenna. Stepped frequency signals making up the Fourier frequency spectrum of the desired synthetic radar pulse are transmitted. In the receiver portion, a digital signal from the microprocessor and a base periodic signal from the master oscillator are both fed to a second frequency synthesizer where the signals are also multiplied to produce a base reference signal. The second synthesizer also includes a quadrature circuit wherein the base reference signal is converted into an in-phase reference signal and a quadrature reference signal. An incoming signal passes from a receiving antenna to an RF amplifier, the output of which is mixed with both the in-phase and quadrature reference signals to provide both in-phase and quadrature output signals. The in-phase and quadrature output signals are then digitized and recorded for each frequency, together with the frequency, until all frequencies have been transmitted and the corresponding return signals received, and a time trace is then reconstructed by inversely transforming the in-phase and quadrature values.
U.S. Pat. No. 4,381,544, issued to Stamm, describes a serial survey technique wherein microwave pulses of several frequencies are radiated to the ground from an antenna on an airborne platform. Part of each radiated pulse penetrates the ground and is absorbed or scattered and reflected by changes in the subsurface dielectric properties at the interfaces between materials having different dielectric properties. A detector, also mounted on the airborne platform, senses the reflected signals and has an empirically determined set of reflection criteria for each potential material interface.
U.S. Pat. No. 4,435,708, issued to Kuriakos, discloses a radio altimeter which uses a triangular modulating waveform for a frequency modulated transmitter. Digital means, synchronized with the triangle wave generator, produces a count gate which is at a high logic level during most of the linear portion of the triangle wave period, and which is at a low level during the portion of the period of the triangle wave near the wave peaks. The count gate and the beat frequency, produced by mixing transmitted and received signals, are applied to logic means which modifies the duration of the high level state of the count gate to produce a derived count gate having a high logic level always of such duration as to equal an integral number of cycles of beat frequency signal. The derived count gate is then used to control a beat frequency counter and a precision clock counter. The outputs of these counters are then arithmetically processed to yield digital altitude information free of step error.
U.S. Pat. No. 4,504,833, issued to Fowler et al., discloses a system similar to that disclosed in the previously referenced Fowler et al. '678 Patent except that, in the receiver portion of the system, the digital signal from the microprocessor and the base periodic signal from the master oscillator are both fed to an offset synthesizer, while only the base periodic signal is fed to a quadrature circuit. The output of the offset synthesizer is mixed with the incoming signal from the receiver antenna and the RF amplifier in a receiver mixer, the output of which is then fed to a power divider where the signal is fed to both a first and a second mixer. The respective in-phase and quadrature signals are also fed to the first and second mixers to provide in-phase and quadrature output signals. U.S. Pat. No. 4,620,192, issued to Collins, describes a radar system wherein a signal from a master oscillator is mixed with a voltage controlled oscillator and the resulting signal is then passed through a filter before being amplified and sent to the transmitter antenna. A portion of the signal being transmitted is also coupled to the receiver to provide a local oscillator signal which is an undelayed replica of the transmitted signal. The incoming signal at the receiver antenna is split and sent to two mixers. The undelayed replica signal is also split and sent to two mixers, after changing the phase of one of the signals by 90.degree.. The signals are heterodyned in each mixer to respectively provide in-phase and quadrature output signals from the mixers. The signals are then sent to a notch filter which will attenuate ground signals and pass any target return signals centered on a Doppler shift frequency differing substantially from a zero frequency. The filtered signals are then digitized and passed to a digital signal processor which includes a digital correlator, a Fast Fourier Transform ("FFT") signal processor, a magnitude processor, a memory, and a constant false alarm rate ("CAFR") processor. The digital signal processor converts the digitized time domain data into a range/Doppler map and reports CAFR threshold crossings with that map to a digital computer.
U.S. Pat. No. 4,670,753, issued to Vacanti, describes a dual channel radar system wherein a transmitted FM signal, which has been circularly polarized in one direction, sweeps a predetermined frequency range. The return signal, which is polarized in both directions, is received by the same antenna, and the received reflections are mixed with samples of the transmitted signal to produce baseband frequency signals on two channels representing, respectively, right- and left-circularly polarized reflections. The signals are processed by an FFT element to produce digitized I and Q signals for each channel. The minimum power measurements for each channel are determined and then compared in order to locate targets in the target area.
However, despite these attempts to improve upon existing commercial radar systems, there has still been a need for a radar system which will distribute the transmitted signal power requirements in a manner which will provide an increase in the system's overall average power, thus resulting in a less complicated and more reliable radar system, as well as providing a increase in the system signal to noise ratio. In addition, there has remained a need for a radar system which is capable of preserving the phase information of the radar return signal and which will take full advantage of digital signal processing techniques, thus providing an increase in the target detection ability of the system. Such a system should also provide for more efficient use of transmitted power by generating signals covering only the frequency range in which a practical antenna radiates most efficiently.
Clearly, it would be desirable to have a relatively uncomplicated and reliable radar system which could provide a high average power signal with a corresponding increase in system signal to noise ratio, and further which would preserve the phase information of the radar return signal for digital processing and analysis of such phase information. To the inventors' knowledge, no such system has existed prior to the present invention and that described and claimed in copending application Ser. No. 07/913,494.